1. Field of the Invention
This invention relates to serial input/output multiplexing apparatus which bi-directionally communicates status and control signals between several remote devices and a programmable logic controller (P.L.C.) over a two-wire asynchronous communication channel and, more particularly, to such apparatus which permits a P.L.C. to control the operation of several remote devices.
2. Description of the Prior Art
In process control techniques, such as used in manufacturing, machine control, telemetry, and the like, a central controller is used to supply control signals to several controllable elements in response to information pertaining to the operation of such elements. In a typical application, a programmable logic controller (P.L.C.) receives status information from remote sensors and produces control information in response thereto which is transmitted to controllable elements. Status information represents two-state information normally monitored in conventional process control. The control information likewise represents two-state conditions of those variables which are controlled in order to regulate the process, or processes, which are carried out. Frequently, the remote sensors and/or the controllable elements are movable with respect to the P.L.C. Heretofore, a separate set of wires extends from each remotely disposed sensor to the P.L.C., and yet another set of wires extends from the P.L.C. to each controllable element.
When separate sets of wires extend between remote devices, whose status are monitored and whose functions are controlled, and the P.L.C., care must be used in wiring the remote devices to the P.L.C. This requires technical skill and is quite time-consuming and laborious. For example, two or three wires must be routed in bundles or wire raceways between each remote device and the P.L.C. In addition to this obvious drawback, the relative movement between the remote devices and the P.L.C., as aforesaid, requires movable assemblies or systems and often is accompanied by fatigue of the wires themselves. As a result, faults in the wire communication channels may occur and this interferes with accurate monitoring and control of the remote devices. Wire faults must, of course, be repaired; and this results in unacceptable "downtime" of the controlled process.
It has been proposed to provide a multiplexing arrangement between a central station and several remote, monitored devices. Typically, however, proposals of this nature are directed to alarm and security systems, whereby signals produced by remote sensors are received at the central station. One example of such a system is manufactured by Ritron, Inc. of Carmel, Ind., under the name "TELESWITCH". This system is a radio telemetry system which transmits up to sixteen contact closures sensed at a remote location to a central station via a radio link over which a digital message is transmitted. It is believed that this TELESWITCH system is not capable of sending control signals back to the remote locations from the central station. Hence, this system appears to be simply a monitoring arrangement.
Another proposed multiplexing system for transmitting monitored information from remote locations to a central station has been suggested by Measurement Technology Ltd., Luton, England, known as System MTL 820. In this switch multiplexer system, the status of up to sixteen remotely located switches or proximity detectors is transferred to a serial bit-stream and transmitted by a single two-core cable to a receiver at the central station. Information from each monitored switch is sampled periodically and then multiplexed onto a single data highway. The System MTL 820 suffers from the same disadvantage discussed above: namely, there is no suggestion or apparent ability to transmit control information from the central station back to any of the remote devices. Moreover, the operating speed of these previous proposals is too slow for many applications wherein sensor information must be updated periodically and quickly to achieve precise control over the conditions being monitored.